Frequency modulation system



Feb. 2,1943. J E. 5mm 2,309,678

' FREQUENCY MODULATION SYSTEM Filed Aug. 31, 1940 2 Sheets-Sheet 1 GROUP GROUP GROUP LOWER .9105 BAND minus/V055 f2, UPPERSIDIEBAAIID FREQUENCIES (CARR/ER] Gkoup N0. 8 w

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ANDDEfl'f' 3 I INVENTOR JAMES E SMITH A TTORNE Y Feb. 2, 1943. J. E. SMITH FREQUENCY MODULATION SYSTEM 2 Sheets-Sheet 2 Filed Aug. .31, 1940 y N5 :5 J

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AV TORNEY Patented Feb. 2, 1943 FREQUENCY MODULATIflN SYSTEM James E. Smith, Brooklyn, N. Y., assignor to Radio Corporation of America, a corporation of Bela- Ware iipplieation August '31, 1940, i Serial No. 354,984

(or. sac-e) 10 Claims.

carrier directly according to the modulating signal amplitude variations. At the receiver, there is employed a single frequency-to-amplitude demodulator or converter having a band Width conversion characteristic at least equal to the total swing or peak-to-peak deviation of the carrier.

-Such'systemsare subject to distortion .due to selective fading-andin fact thisdistortion is-sufiiciently severe to.preclude the use of frequency modulation transmissions at normal ShOIt-WELVB long-distance radio communication. circuits.

The system of the ,present invention, broadly speaking, overcomes the foregoing defect by breaking up or segregating the transmitted signal. siderbands into individual. frequency groups,

each group of which contains :all of the necessary intelligence, and then receiving and detecting certain ones of thesegroupsof side bands separately. Thereceiver outputs of the system of the present invention are .then' combined in accordance with the usual diversity receiving methods. The effects of selective fading are'thus minimized.

A better understanding of the invention may be had by referringlto the followingdescription in conjunctionwiththe accompanying drawings, wherein:

Fig. 1 illustrates graphically desirable ,side band groups which can be-selected from a .particular table andused for the purpose of the invention;

Fig. 2 illustrates one form ofreceiving systel. which may be used in the practice of the invention; and

Fig. 3 illustrates diagrammatically onemeans for transmitting signals in accordance .with the invention.

For purposes of expositiomthe principles underlying the presentinvention will now be described.

It is known that a frequency modulation radio frequency carrier, when modulated by a single sub-carrier tone, is equivalent to a carrier and an infinite number of sinusoidal primary side band carriers above and below the radio frequency carrier and having frequency spacings equal to the sub-carrier frequency. Normally, the side band components greatly removed from the radio frequency carrier are of small amplitude and may be neglected. The spreaid of the pertinent side band carriers, 1. e., those having considerable amplitudais a function of the modulation index (ratio of peak radio frequency carrier deviation to sub-carrier frequency in this case). If the sub-carrier frequency is now varied,

each of the foregoing primary side bands will vary in frequency indirect correspondence with the variation' of the sub-carrier, Each ofthese varying primary side band carriers is actually composed of a group of frequency components including a secondary sub-carrier and secondary upper and lower side bands, determined by the modulating signal intelligence. The position of the primary side band carriers relativeto the radiofrequencycarrier is determined by the frequency of thesub-car'rien-and-the amplitude of the primaryside band carriers is determined by the amplitude of the sub-carrier. The-position of thesecondaryside bands with respect to the secondarysub-carrier in each group is determined by the frequency components of the intelligence, and the amplitude of these secondaryside bands is determined bythe amplitudeof the frequency components of the intelligence. However, theamplitudes of thevarying primary side band carriers constituting each group is afunction of the'modulationin'dex and for a particular amplitude of the sub-carrier certain-primaryside band carriers may have their amplitudes decreases to zero or to a reverse polarity when the frequency of the sub-carrier is varied. Such side band groups whose amplitudes decrease to'zero cannot be satisfactorily limited. Other side band groups, on the other hand, will havesufiicient amplitudes for-a fixed modulation index' to permit satisfactory limiting. These last wave groups may be demodulated separately and in accordance with the invention then combined in the usual diversity receiving fashion. Selective fading maysubmerge one entire signal group in the noise, but if three or more groups are employed in the diversity system of the invention, the probability that they will all fade simultaneously is extremely small. Following the principles just described, the present invention achieves communication by frequency modulation by varying or shiftingthe sub-carrier frequency in accordance with the intensity or amplitude of the modulating signal. v

To illustrate a specific embodiment of the invention, given by Way of example only, let us suppose that a carrier in the frequency modulation transmitter is modulated by a one kilocycle tone and th amplitude of the tone deviates the radio frequency carrier plus and minus ten kilocycles. The modulation index which, as mentioned before, is the maximum devivation (to one side of the carrier) divided by the maximum audio frequency transmitted is then m=10 and the resulting side bands are shown in the table given below. This table shows different sideband amplitudes and phases for three different modulation indices. If the tone frequency of one kiloeycle is now changed to 900 cycles, but its amplitude maintained constant as before and of the same value, the modulation index will then be m=11.1. Further, if the frequency of the tone is changed to 1100 cycles without varying its amplitude, the modulation index becomes 172:9.08. The following table shows the side band amplitudes and phases (plus and minus) for these three conditions which represent a case where the sub-carrier frequency is frequency modulated from 900 cycles to 1100 cycles by the signal intelligence.

recorder or other translating devices.

Referring to Fig. 2, the upper and lower side bands of the selected groups 2 and 8, taken as an illustration from the foregoing table, are individually collected by antennas l, 2, 3 and 4 which are located at geographically spaced points where the signal does not fade similarly. Observations have shown that the receiving antennae need only be separated from one another by a distance of a few hundreds of feet, and less, in order to get Groups I00") 110'") J") J") 40 150) I110") J1(m) J's(m) If the 900 to 1100 cycle tones in the foregoing this effect. These antennae may be polarized example are the extreme or peak deviation fre 35 in the same plane or in different planes, and if quencies of an actual frequency modulating sysdesired, be of the steerable directive type in order tem in accordance with the invention, it will be to insure optimum reception. The radio fre evident that as the sub-carrier tone varies in frequency voltages from the antennae I, 2, 3 and 4 quency between these limits in accordance with are respectively fed to the inputs of radio fre the intensity or amplitude of the modulating sigquency amplifiers and detectors 5, 6, I and 8 nal there will be instantaneous side bands lying between the extreme values for the individual frequency groups given in the table.

In accordance with the invention, one or more of those side band groups (such as numbers 2, 5 and 8 of the table) which will permit limiting, are detected individually and then combined. Side band groups such as numbers 1, 4 and 7 of the above table are not used since the side band amplitudes in these groups become zero during continuous sub-carrier frequency modulation. By selecting channels or groups such as numbers 2 and 8 of the above table, the degree of amplitude modulation of the radio frequency side bands is wellwithin the operating range of normal limiters. Both the upper and lower side band groups 2 and 8 could be used individually in the receiver of the invention. Further, the selected groups for detection are well separated in the frequency spectrum so that selective fading would not normally affect all groups simultaneously. Selective fading of the radio frequency carrier or any of the components outside the chosen bands thus does not destroy the received signal. This is one important advantage which the present invention has over frequency modulation systems employing a common detector for the entire radio frequency band.

Fig. 1 shows the desired side band groups or channels to be detected and combined. These are shown without regard to the actual phases of the components since only the relative phases within a group are of importance. In each group there are shown a solid vertical line and two slightly smaller vertical dash lines on opposite sides of the central solid line. The solid line where they are converted to an intermediate froquency and passed on to suitable intermediate frequency amplifiers 9, 9. The receiver constitut ing radio frequency amplifier and detector 5 is selective by means of filters or other suitable 4.3 circuits to the lower side band frequencies in group 8, while the receiver constituting the radio frequency amplifier and detector 6 is selective to the upper side band frequencies of group 8; Similarly, the receivers constituting amplifiers and detectors 1 and 8 are respectivelyselective to the lower and upper side band frequencies of group 2. frequency amplifiers 9, 9 may, if desired, be reduced to a still lower intermediate frequency be- ;1-3 fore being individually passed on to limiters 10, m. The effect of the limiter in is to yield a constant amplitude output regardless of the an1- plitude variations of the voltage fed into the limiter.

The outputs from the intermediate The output from each limiter 10 is w passed to an analyzing detector or so-callecl fre quency modulation-to-amplitude modulation discriminator circuit and detector II, which converts the frequency modulation signals to amplitude modulated signals of an audio frequency 05 character to which the recorder or translator I2 1s responsive. These amplitude modulated signals are a true reproduction of the original frequency modulation. Analyzing detector H may be of the balanced type shown in Seeley Patent No. 2,121,103, granted June 21, 1938, and in Usselman Patent No. 1,794,932, granted March 3,. 1931; or an unbalanced detector operating on one side of the operating range of frequencies, as shown in Fig. 3 of Hansell Patent No. 1,803,504,.

'1'5 granted May 5, 1931.

Translator l2 may be a diversity receiver-"of Fig; 2 be provided with 'circuit arrangements for selecting for ultilization in the translator I2 that'receiver 'ofthe diversity receiving system giving" the best results/while at the same time those receivers 'which-have'received feeble or unsatisfactory 'signals are eifec- 'tively ignored or disassociatedfr'om the "translator l2.

Such a circuit arrangement may constitute a suitable receiver control means l'abeled l3. Reference is herein made to United St'ates'Patents Nos. 1,888,065, granted November 1932, to H. H. Beverage, and 1,913,428; granted June' 13, 1933 to E. Bruce, showing circuits for selecting the strongest signal for utilization in a diversity system.

Fig. 3 illustrates one type of transmitterwhich can be employed in the practice of the invention. The various operations have beendivided *into three stages, and below the block diagrams representing the various stagesI haveshown the fre quency characteristics which pass through each portion of the equipment. Thetransmitter includes a suitable signal 'amplifier' 2l to whose input is applied the signal intelligence having any suitable range of frequencies from zero to fm. The output of the signal amplifier is then applied to apparatus 22 representing a frequency modulator and a variable frequency sub-carrier generator. The sub-carrier frequency is represented by is and the limits to which the subcarrier is modulated is represented by :Afs. The modulation index is given by the formula fm The output of apparatus 22 is used to frequency modulate the radio frequency transmitter 23. This radio frequency transmitter includes a suitable radio frequency carrier generator whose frequency is represented by fc. The output of apparatus 23 which is radiated from antenna 24 includes a plurality of frequency modulated carriers each of which is composed of a group of side bands, each group containing the same intelligence. These frequency modulated carriers are represented by the characters fc+fs, fc-I-2fs, fc-l-3 s, on one side of the radio frequency carrier f0, and frequency modulated carriers fcfs, fc-2fs, fc3fs On the other side of the radio frequency carrier ft:-

In the transmitter of the invention it is also contemplated that, if desired, those side band groups which extend beyond the useful range may be suppressed in order to reduce interference on an adjacent radio frequency channel.

Another advantage of the present invention is that it increases the range of reliable communication by frequency modulation way beyond the optical range. The invention enables the use of communication by frequency modulation on carrier frequencies which reach and are reflected from the ionosphere, such as those now used on long distance short wave radio circuits.

What is claimed is:

1. The method of communicating which includes modulating the frequency of a carrier by a sub-carrier, shifting the frequency of the subcarrier in accordance with the amplitude of the modulating signal to produce a wave having a plurality of spaced frequency modulated carriers each composed of a group of side bands containing-the same intelligence, radiating t-he said frequency modulated carriers, receiving separately from-one another certain ones of said frequency 'modulated carriers, separately limiting and detecting said received frequency modulated car- -r'iers,--and combining the resultant detected eni ergies.

'' containing the' same intelligence, which includes -2.- The method of receiving a frequency modulated wave having a plurality of frequency modulated side-bandson-both sides of said waveand receiving "separately from one another certain ones'of said frequency modulated side bands whose amplitudes do not decrease tozero; separately limiting'and detecting said received frequency modulated side bands and combining the resultant detected energies.

' 3. The method of'receiving a frequency modulatedwave having a plurality of frequency modulated side bands on'both sides of said wave and containing the same intelligence, which includes receivingseparately from one another at least two frequency modulated side bands whose amplitudes do not decrease to zero, separately amplifying, limiting and detecting said received frequency modulated side bands, and translating the resultant detected energy from that frequency modulatedsideband having the largest voltage.

4. In a frequency modulation system, means for modulating the frequency of a carrier by a sub-carrier, means for shifting the frequency of the sub-carrier in accordance with the amplitude of the modulating signal to produce a plurality of spaced frequency modulated carriers each composed of a group of side bands containing the same intelligence, means for radiating said frequency modulated carriers, means for receiving certain ones of said frequency modulated carriers separately from one another and for separately limiting the amplitudes thereof, means for separately detecting the resultant energies, and signal translating means responsive to said detected energies.

5. In a frequency modulation system wherein means are provided for modulating the frequency of a carrier by a sub-carrier, and means are provided for shifting the frequency of the subcarrier in accordance with the amplitude of the modulating signal to produce a plurality of spaced frequency modulated carriers each com-" posed of a group of side hands containing the same intelligence, there also being provided means for suppressing certain undesired frequency modulated carriers, and means for radiating the non-suppressed frequency modulated carriers, a receiving system having a plurality of differently selective receivers for passing the received frequency modulated carriers, and a common utilization circuit for said receivers.

6. In a frequency modulation system, means for modulating the frequency of a carrier by a sub-carrier, means for shifting the frequency of the sub-carrier in accordance with the amplitude of the modulating signal to produce a plurality of spaced frequency modulated carriers each composed of a group of side bands containing the same intelligence, means for radiating said frequency modulated carriers, means for receiving separately from one another certain frequency modulated carriers whose amplitudes do not become zero, means for separately limiting the amplitudes of said received selected frequency modulated carriers, means for separately detecting the resultant energies, and signal translating means responsive to said detected energies.

'7. The method of communication which includes modulating the frequency of a carrier by a sub-carrier, shifting the frequency of the subcarrier in accordance with the amplitude of the modulating signal to produce a wave having a plurality of spaced frequency modulated carriers each composed of a group of side bands containing the same intelligence, radiating the said frequency modulated carriers, receiving separately from one another certain frequency modulated carriers whose amplitudes do not become zero,

separately limiting and detecting said received frequency modulated carriers, and utilizing the detected energies.

8. The method of overcoming selective fading in radio communication which includes modulating the frequency of a carrier by a sub-carrier, shifting the frequency of the sub-carrier in accordance with the amplitude of the modulating signal to produce a wave having a plurality of spaced frequency modulated carriers each composed of a group of side bands containing the same intelligence, radiating the said frequency modulated carriers, receiving separately from one another certain ones of said frequency modulated carriers, and utilizing said received carriers.

9. In a frequency modulation system for overcoming the efi'ects of fading, means for modulating the frequency of a carrier by a sub-carrier,

means for shifting the frequency of the sub-carrier in accordance with the amplitude of the modulating signal to produce a plurality of spaced frequency modulated carriers each composed of a group of side bands containing the same intelligence, means for radiating said frequency modulating carriers, 2, receiving system having separate antennas for collecting certain ones of said carriers, and means coupled to said antennas for impressing the intelligence of one or more of the received carriers upon a signal translation circuit.

10. The method of communication which includes modulating the frequency of a carrier by a sub-carrier, shifting the frequency of the subcarrier in accordance with the amplitude of the modulating signal to produce a wave having a plurality of spaced frequency modulated carriers each composed of a group of side bands containing the same intelligence, radiating the said frequency modulated carriers, receiving certain ones of said frequency modulated carriers, separately limiting and detecting said received frequency modulated carriers, and combining the resultant detected energies.

JAMES E. SMITH. 

